In a large bowl, add the all purpose flour, unsalted butter, and boiling water. Let it sit for a few seconds to melt the butter, then stir until thick and pasty.
Add sugar (or honey), buttermilk, egg, and sourdough starter. Stir with a whisk until well mixed.
Add the remaining all purpose flour (or bread flour) and salt. Stir until too difficult to mix with a fork, then use your hands to bring the ingredients together into a cohesive, smooth dough.
Grease the bowl and cover with a non-permeable covering. Rise at room temp for 12 hours, or until double.
On a floured surface, gently stretch and pat the dough into a rectangle about the width of your loaf pan. Shape the loaf as preferred.
Place the loaf in a greased loaf pan. Cover and rise for 2 hours or until double.
Bake on the center rack of a preheated 375F oven for 50 minutes, or until the internal temperature is 195F / 91C.
Cool fully before slicing, ideally 8 hours.